The goals of the research remain as proposed in HL 12758-11. The studies are aimed at the understanding of the interplay of various factors controlling the secretion and biosynthesis of aldosterone. We have accumulated evidence that potassium is an important intermediate in a number of factors that regulate aldosterone secretion. Entrance of potassium into cells seems to be the critical feature. In humans prostaglandin A is increased along with renin by sodium depletion. Although prostaglandin A has been shown by others to stimulate aldosterone secretion, our studies are still in progress to determine whether it plays an aldosterone stimulating role in man. In the spontaneously hypertensive rat and normotensive Wistar rat, we found a suppressed plasma renin activity as the rats aged. Suppressed response to sodium depletion was not present in the Sprague- Dawley rats, as they aged. We are studying the mechanism of the suppressed renin activity. In vitro studies on the isolation and physiological importance of a new adrenal malic enzyme are being carried out.